Engineers are constantly seeking ways to reduce undesirable engine emissions. One strategy is to seek ways to improve performance of fuel injection systems. Over the years, engineers have come to learn that engine emissions can be a significant function of injection timing, the number of injections, injection quantities and rate shapes. However, it is also been observed that an injection strategy at one engine operating condition may decrease emissions at that particular operating condition, but actually produce an excessive amount of undesirable emissions at a different operating condition. Thus, for a fuel injection system to effectively reduce emissions across an engine's operating range, it must have the ability to produce several different rate shapes, have the ability to produce multiple injections and produce injection timings and quantities with relatively high accuracy. Providing a fuel injection system that can perform well with regard to all of these different parameters over an entire engine's operating range has proven to be elusive.
Apart from addressing rate shapes, timing accuracy and quantity accuracy, etc., other issues should be addressed. For instance, in order to be commercially viable, fuel injection systems should not only exhibit superior performance but should also provide for efficient operation. In addition, there should also be the ability to mass produce fuel injection system components, such as unit injectors, with acceptable performance deviations from one another. Thus, in order to present a commercially viable fuel injection system it should satisfy stringent emissions requirements, address a number of problems associated with a relatively wide array of performance capabilities combined with acceptable injector to injector performance variations, and further should exhibit competitive operational efficiencies.
One apparent attempt to satisfy at least some forthcoming performance demands is disclosed in “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messrs. Bernd Mahr, Manfred Dürnholz, Wilhelm Polach, and Hermann Grieshaber, Robert Bosch GmbH, Stuttgart, Germany, at the 21st International Engine Symposium, May 4-5, 2000, Vienna, Austria. This reference teaches a common rail system and a directly controlled fuel injector that purportedly has the ability to inject medium pressure fuel directly from the rail, or utilize the common rail to pressure intensify fuel within the injectors for injection at relatively high pressures. While this system may have the ability to exhibit some improved performance characteristics, it appears to suffer from a number of drawbacks. Among these are the fact that the system relies upon circulating medium pressure fuel around an engine and always maintaining the fuel injectors in a pressurized state, which results in continuous leaking and efficiency degradation. In addition, the system appears to employ a two way direct control needle valve that temporarily opens the high pressure rail directly to the drain via a flow restriction during each injection event, resulting in a substantial amount of fuel being expended for no apparently useful purpose. The Bosch system likely suffers from other drawbacks, but most of those limitations lie hidden due to the limited disclosure of the system at this time.
The present invention is directed to one or more of the problems set forth above.